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NBME 21 Answers

nbme21/Block 1/Question#28

A 47-year-old man is brought to the emergency ...

Mixed venous oxygen tension

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submitted by nicnac20(6),

Mixed venous oxygen tension is the amount of oxygen bound to hemoglobin as it returns to the right side of the heart after traveling to the tissues.

Decreased oxygen delivery to the tissues due to decreased cardiac output leads to the tissues extracting more oxygen from the passing RBCs than they normally would, which decreases the overall mixed venous oxygen tension.

pparalpha  A helpful equation is CO=rate of O2 consumption/(arterial O2 content-venous O2 content): Fick principle If CO is decreased, then the difference between arterial O2 content and venous O2 content is increased +2  

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submitted by paloma(2),

What about pulmonary vascular resistance? It follows the systemic vascular resistance?

cooldudeboy1  crackles are heard bilaterally so there is pulm patho which leads to increased pulm vascular resistance, since systemic blood flows into the lungs. any block in the flow ahead (lungs) will increase resistance in flow behind ( systemic ) +  

I get why the mixed venous oxygen tension decreased. However,, isn't the systemic vascular resistance also decreased?

yb_26  no, decreased CO => peripheral vasoconstriction => SVR will be increased +2  
yssya1992  No SVR will increase due to RAAS and SAN thats why we decrease afterload in HF treatment ( ACEI, ARBs ) +2  
snafull  Wouldn't pulmonary vascular resistance also be decreased here due to pulmonary vasodilation in the setting of an MI? +  
cienfuegos  @snafull: my initial thought is that we would see pulmonary vasoconstriction because of the relatively low oxygen tension (that results from the low cardiac output). +1  

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submitted by hayayah(445),

Mixed venous oxygen saturation (SvO2) is measured in the pulmonary artery. SvO2 samples the true mixed venous blood leaving the right heart. Measurement of mixed venous oxygen saturation (SvO2) from the pulmonary artery has been advocated as an indirect index of tissue oxygenation.

In cardiogenic shock you have decreased CO --> decreased O2 delivery --> decreased SvO2.